Understanding Users
Following are the types of users that normally come in a Linux OS.
1. Administrator or Super User
2. Normal Users
3. Sys Users or Pop Users (Users that have no shell to log in into)
4. Network User
The information regarding users is normally stored in the file /etc/passwd.
Each entry in the passwd file has 7 fields as shown:
Username : X : UID : GID : GECOS : Home directory : Default shell
Username - gives the name of the user
X - indicates whether the user has a password
UID - the User ID ( 0 is for root user , 1-499 are pop users, greater than 500 are normal users)
GID - the Primary Group ID number
GECOS - It is used to give comment or description for that user
Home Directory - Specifies the home directory of the user
Default Shell - Specifies the default shell the user gets
Each user has a primary group associated with it. When a user is created, a primary group of the user with the same name as that of the user is created.
Information regarding the group is contained in the field /etc/group
Each entry in the group file contains 4 fields :
Groupname : X : GID : Members in the group
Adding Users
useradd <username> - Creates a user
passwd <username> - Sets a password for the user
useradd -d <directory location> <username> - Creates a user and makes the directory specified, the user's home directory.
useradd -s <shell> <username> - Creates a user by specifying the default shell the user gets.
Modifying Users
usermod -g <groupname> <username> - Change primary group of the user
usermod -G <groupname> <username> - Change secondary group of the user
User Groups
groupadd <groupname> - Creates a group
gpasswd <groupname> - Set or change the password of a group
groups <username> - Lists the groups of a user
groupdel <groupname> - Deletes a group
Delete Users
userdel <username> - Deletes a user
userdel -r <username> - Deletes a user along with the home directory of the user.
" To create a pop user - useradd -s /sbin/nologin <username> "
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